Micromechanical optical switch

ABSTRACT

The present invention relates to optical MEMS components, and in particular, to a micromechanical optical switch. A removable layer is used during fabrication to define the gap between optical waveguides and a moveable element in the form of a mirror that is moved between states. This provides a high speed, low-power optical switch that is readily manufacturable.

RELATED APPLICATIONS

[0001] This application is a divisional of application Ser. No.09/645,203 filed Aug. 25, 2000. The entire teachings of theabove-referenced application are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] Optical switches have been developed using guided wave devices orfree-space mechanical devices. Guided wave devices use waveguides,whereas free-space devices use optical beams in free space with movableoptical elements such as mirrors or lenses.

[0003] Guided wave devices typically divert light from one arm of thedevice into the other by changing the refractive index of one of thearms of the device. This is typically done using electrical, thermal, orsome other actuating mechanism.

[0004] The free-space approach has an advantage over the guided-waveapproach in some applications. It has very low cross talk because thewaveguides are physically isolated from one another and coupling cannotoccur. The principal source of cross talk in this approach is scatteringoff the movable optical element. In addition, free-space devices arewavelength-independent and often temperature-independent.

[0005] Existing designs employ mirrors positioned at the intersection ofinput fibers and output fibers. Due to the spreading of the light beamas it leaves the waveguide and travels toward the mirror large mirrorsare used that require mounting and angular placement accuracy. There canbe significant difficulty in actuating such a relatively large structurequickly and accurately at the switching speeds required for opticalcommunication systems.

[0006] Thus, a need exists for an optical switch having the advantagesof the free-space approach, without the disadvantage of existingdesigns.

SUMMARY OF THE INVENTION

[0007] The present invention relates generally to the field of opticalMEMS (microelectro-mechanical system) and more specifically to the useof fabrication techniques used in making micromechanical devices tofabricate high speed optical MEMS for optical communication networks.The method employs the use of a removable layer that is formed betweenan optical waveguide and a movable switch element that has been formedover the removable layer and the waveguide. The removable or sacrificiallayer is preferably formed using a conformal material such as parylene(poly-para-xylene).

[0008] A preferred embodiment of the invention can use a silicasubstrate with optical waveguides formed therein as the initialstructure in the manufacture of the optical MEMS. After formation of atrench in the substrate to define a gap between waveguide elements, afirst mask is used to define the routing wire geometry on the uppersurface of the substrate. The trench can have a width of about 3 to 20μm. Subsequently the removable layer is formed followed by the use of asecond mask for fabrication of a switch element layer.

[0009] After patterning of the switch element layer, a spacer layer,preferably a photoresist layer, is spun on the surface and patternedusing a third mask. A metallization layer, preferably an evaporatedlayer of copper, is deposited and a further photoresist layer is formedusing a two mask exposure sequence. This defines a mold for fabricationof a plating layer. In a preferred embodiment of the invention, nickelis electroplated into the mold to form an integral electrode structure.

[0010] The removable layer is then preferentially etched to release theswitch element which has been fabricated with a spring that supports theswitch relative to the substrate. The characteristics of the springdefine the speed and pull-up voltage of the switch element.

[0011] The electrodes are used with an overlying actuating electrodestructure to actuate movement of the switch element between states. Apreferred embodiment of the invention uses a reflective element ormirror that is moved from a first position, in which light from a firstwaveguide is reflected by the mirror into a second waveguide, to asecond position in which the mirror is translated vertically to permitlight to pass through the gap on a linear optical path into a thirdoptical fiber that is aligned along a single axis with the first fiber.The waveguides and/or the trench can be filled with air or in anotherembodiment can be filled with a fluid. Further details regarding the useindex matching fluids are described in International Application No.PCT/US99/24591 filed on Oct. 20, 1999, the entire contents of which isincorporated herein by reference.

[0012] Another preferred embodiment of the invention involves thefabrication of an array of switches on a single substrate that can serveas a monolithic array, or alternatively, the substrate can be diced toprovide separate switches or arrays of switches.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGS. 1A-1K illustrate a method of fabricating an optical switchin accordance with the invention.

[0014]FIG. 2 is a schematic cross-sectional view of a preferredembodiment of an optical switch in accordance with the invention.

[0015]FIG. 3 illustrates the switching time of a preferred embodiment ofthe invention.

[0016]FIG. 4 is a top view of an optical switch in accordance with theinvention.

[0017]FIG. 5 is a top view of another preferred embodiment of theinvention.

[0018]FIG. 6 is a top view of another preferred embodiment of theinvention.

[0019]FIG. 7 is a top view of another preferred embodiment of theinvention.

[0020]FIG. 8 is an array of optical switches made in accordance with theinvention.

[0021] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] A preferred method of fabricating a micromechanical opticalswitch is illustrated in the process sequence of FIGS. 1A-1K. Thismethod begins with a substrate 10, such as silica wafer, in which acavity or trench 12 has been formed by standard etching techniques. Asdescribed in detail below, the substrate can include one or more opticalwaveguides. A first mask pattern is formed for conductive routing linesby depositing a photoresist layer 14 and selectively removing portionsthereof to define the metallization pattern 16,18.

[0023] In FIG. 1B, a conductor layer 20 is formed, preferably byevaporation of a metal. In a preferred embodiment, a plurality of layersincluding a titanium layer, a nickel layer and a gold layer are used toprovide the desired electrical and mechanical properties. In an example,the titanium layer has a thickness of 500 Å, the nickel layer has athickness of 1500 Å and the gold layer has a thickness of 3000 Å. Arinse can be used to remove excess metal and the photoresist andoverlying metal is than removed to provide conductive routing lines 30shown in FIG. 1C.

[0024] A sacrificial layer 40 is then formed on the device, preferably alayer of parylene having a thickness in the range of 0.5 μm to 25 μm,depending on the width of the trench 12. This is followed by a layer 42,of a reflective material such as gold. In this particular example theparylene layer has a thickness of 3.5 μm and the gold layer is depositedin 0.2 μm steps for a total thickness of about 2.0 μm.

[0025] In FIG. 1E, the reflective layer 42 is patterned to form areflector or mirror 50. A photoresist (AZ9260) is spun, baked, exposed,and developed to define the mask pattern 55 and the exposed gold isremoved with a Transene TFA etchant. Next, after removal of the mask, adirectional (RIE) etch in an 0₂ plasma is used to remove the parylenewith the mirror 50 acting as a mask and leaving a residual layer 62 asseen in FIG. 1F.

[0026] Another photoresist layer 70 is then deposited and patterned todefine anchor positions 72 (FIG. 1G). Layer 70 has a specified thicknessto define a gap between the suspended structure that will support themirror relative to the substrate. The gap is preferably in a rangebetween 5 and 20 μm and in this particular example is about 15 μm. Thesize of the anchor openings can be measured to verify proper alignmentand preferably each opening has an area in a range between 40 and 50μm².

[0027]FIG. 1H illustrates formation of a metal seed layer having athickness in a range of 1000 to 50,000 Å. In this particular example, acopper layer having a thickness of 5000 Å is deposited by evaporation.

[0028] Another photoresist pattern 90 is formed as shown in FIG. 1Iusing a digitally controlled oven at 45° for 4 hours. A two stepexposure sequence is used to minimize variations in thickness of thephotoresist. Features 92 of the photoresist layer 90 are used to defineelectrodes in the suspended membrane that are used in actuating movementof the switch element. As shown in FIG. 1J, a nickel layer 100 isformed, preferably using an electroplating process in which threeseparate regions, the first region 102 being at the anchor, the secondregion at electrodes 104, 106 and the third region at the mirror 108. Ina preferred embodiment of the invention a nickel sulfate solution isused with a current of 17.5 mA at 45° C. with a plating time of 30minutes to provide a 6 μm thick layer.

[0029] As shown in FIG. 1K, the photoresist 90, 92 is removed, theexposed copper is then etched using ammonium hydroxide and copper (II)sulphate to access the spacer material 70 which is then removed.Finally, a diclorobenzene etch is performed at 150° C. that removes theremaining parylene 61 to release the mirror structure 120.

[0030] The above procedure can also be used in fabricating a mirror thatcan be displaced laterally in the trench using a different method ofactuation such as electrostatic comb drive or thermal actuation whichcan be used to provide a bistable switch, for example.

[0031] Illustrated in the schematic cross-sectional view of FIG. 2 is anoptical switch 200 in accordance with the invention. An overlyingactuating electrode panel 202 having an actuating electrode 204 that isseparated from the suspended membrane 208 by a gap 218 that ispreferably about 50 μm. Spacers 206 can be made using an oxide toprevent shorting between electrodes 204 and 226 or the mirror surface210.

[0032] Note that optional pull down electrodes 216 can also bepositioned in the gap 220 between the fiber cladding 224 and themembrane 208 that is preferably about 15 μm. The substrate has athickness 222 that includes the cladding 224 surrounding the fiber core212. The fiber core is preferably about 6 μm. The mirror includes theswitching element 214 that moves vertically within the trench 228. Thepitch of the membrane structure is in a range of 100 to 2000 μm, and ispreferably about 500 μm. The upper panel 202 can be electricallyconnected to the lower substrate system using flip chip bonding oreutectic bonding. The driver circuit for the switch can be mounted onsubstrate or packaged separately.

[0033]FIG. 3 graphically illustrates the vertical mirror displacement asa function of time for three different pull-up voltages as a function oftime.

[0034] FIGS. 4-7 illustrate preferred embodiments of the spring systemthat supports the membrane relative to the substrate. The spring isconfigured to provide a vertical displacement of between 20 and 30 μm.Generally, a higher spring constant in the range of 1.0 to 4.0 N/m alongwith a higher operating voltage in the range of 50-150V results in afaster response time. It is also desirable to minimize or eliminaterotation of the membrane during displacement. FIG. 4 illustrates aspring system 300 having four beams 302 extending from anchors 304 to amembrane connection 306. This particular embodiment has a stiffness of1.99 N/m, a rotation of 0.0025 degrees and a spacing of 10 μm. Thespring system 320 of FIG. 5 has four spring elements 322 extending fromanchors 324 to membrane connectors 326. This embodiment has a higherstiffness at 4.1 N/m, a smaller rotation at 0.0015 degrees and a 10 μmspacing. The embodiment 340 of FIG. 6 has four spring elements 342connected at anchors 344 and connected at 346. The system has astiffness of 2.8 N/m, a rotation of 0.002 degrees and a smaller spacingat 5 μm. The system 360 of FIG. 7 has stiffness of 4.49 N/m, no rotationand a 10 μm spacing.

[0035]FIG. 8 illustrates an array 400 of switches fabricated inaccordance with the invention. The array can be 8×8, 32×32, 64×64 or anyother desired configuration as needed for a particular application. Inthis particular embodiment an 8×8 array having input fibers 402, adiagonally positioned array of switch elements that either reflect lightfrom the input fibers to output fibers 406, or allow light to passdirectly through the trench to output fibers 408. The output fibers 406can be orthogonally arranged relative to fibers 402, or they can bearranged at some other oblique angle.

[0036] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An optical switch comprising: a substrate having a first optical waveguide having an output surface and a second optical waveguide having an input surface, the input surface and the output surface being separated by a cavity; an optical switch element positioned within the cavity and moveable between a first position and a second position; and a spring mounted actuator that moves the optical switch element between the first position and the second position to control transmission of light between the first optical waveguide and the second optical waveguide.
 2. The optical switch of claim 1 further comprising a third optical waveguide coupled to the cavity such that the switch element couples light from the first waveguide in the first position.
 3. The optical switch of claim 1 further comprising an array of input waveguides and an array of output waveguides, each pair of input and output waveguides having an optical switch element.
 4. The optical switch of claim 1 wherein the substrate comprises a silica wafer.
 5. The optical switch of claim 1 wherein the optical switch element comprises a reflector.
 6. The optical switch of claim 1 wherein the spring comprises a plurality of spring elements attached at a first end to the substrate and at a second end to a moveable beam.
 7. The optical switch of claim 1 wherein the spring has a stiffness between 1.0 and 5.0 N/m.
 8. The optical switch of claim 1 wherein the switch element moves from the first position to the second position in less than 1 ms.
 9. The optical switch element of claim 1 further comprising a control circuit that actuates movement of the switch, the circuit operating at a voltage in a range between 50 to 300V. 